1. Field of the Invention
The present invention relates to a paper folding apparatus for folding printed sheets of paper discharged out of a printing machine, such as a copying machine.
2. Discussion of the Related Art
In a general copying machine, a sheet of recording paper flows through the machine in the following way. The sheet, which is fed from a paper supply cassette into the copying machine, first reaches an image forming stage for transferring a latent electrostatic image on the sheet. The sheet bearing the latent image is transported to a transfer/fusing stage where the latent image is transferred and fused onto the sheet. The sheet with the fused image is discharged from the copying machine into a paper receiving tray, and stored therein.
In a copying machine capable of copying original documents of large size, drawings or graphic images are frequently copied on recording sheets of paper of large size, for example, A0 or A1. Generally, the copied, large sheets are manually folded to have the size easy to handle and store. The manual folding work is troublesome and time consuming. Frequently, it takes time period several times as long as a copying time.
To cope with this, there have been proposed automatic folding apparatuses in use with copying machines. For those apparatuses, reference is made to Japanese Patent Unexamined Publication Nos. Sho. 46-5909, 49-8319, and 61-226461, for example.
The mechanical construction of a conventional paper folding apparatus is schematically illustrated in FIG. 1. As shown in FIG. 1, the paper folding apparatus 10 includes a feed roller pair 11 consisting of a drive roller 11a and a pinch roller 11b, a first folding roller pair 12 consisting of a drive roller 12a and a pinch roller 12b, and a second folding roller pair 13 consisting of a drive roller 13a and a pinch roller 13b. Under the feed roller pair 11, the first and second folding roller pairs 12 and 13 are disposed symmetrically with respect to the vertical line extended down from the feed roller pair 11. The apparatus 10 further includes a paper guide 15 for guiding a sheet of paper 14 fed from the feed roller pair 11 selectively to the first or second folding roller pair. The paper guide 15 can be swung about a shaft 15a by means of a drive mechanism (not shown). A paper guide plate 16, further provided in the apparatus 10, is disposed between the first and second folding roller pairs 12 and 13. The guide plate 16 selectively guides the sheet of paper 14 toward the first folding roller pair 12 or the second folding roller pair 13.
A feed roller pair 18 consisting of a drive roller 18a and a pinch roller 18b feeds the sheet of paper 14 through a paper feed path 17 to between the rollers of the feed roller pair 11 in the paper folding apparatus 10. The sheet 14 is fed into the paper guide 15 by means of the feed roller pair 11. Then, it is selectively guided, by means of the paper guide 15 to the first folding roller pair 12 or the second folding roller pair 13. In the illustration of FIG. 2, the paper guide 15 is going to guide the sheet 14 to between or the nip of the paired rollers of the second folding roller pair 13. When the first and second roller pairs 12 and 13 repeat the forward and reverse turns, the paper guide 15 swings about the shaft 15a. The leading edge of the sheet 14 slides along the upper surface of the guide plate 16 to alternately be applied to the paired rollers of the roller pairs 12 and 13. In this way, the sheet 14 is progressively folded a predetermined number of times.
In order to fold a large sheet of paper 14 of size A0, for example, to have the size of A4, two paper folding sections each having the construction like that of the paper folding apparatus 10 are used; a first folding section folds the sheet of paper in a first direction, and a second folding section further folds the folded sheet in a second direction at a right angle to the first direction. Examples of the sheets 14 folded by the first folding section, just before they are put into the nip of the second folding section, are illustrated in FIGS. 3(A) and 3(B). In the second folding section, the folded sheet 14 is put into the nip of the second folding roller pair designated by reference numeral 13 in the paper folding apparatus 10.
The paper folding operation in the second folding section will be described with reference to FIG. 2. The leading edge of the sheet of paper 14 is first put into the nip of the first folding roller pair 12, as indicated by a solid line. Then, the paper guide 15 turns toward the second folding roller pair 13, while at the same time the rollers of the first folding roller pair 12 turn in the directions of arrows. As the result of the paper guide turn and the roller turn, the sheet of paper 14 is moved toward the second folding roller pair 13 to be shaped as indicated by a dotted line. The folded sheet thus shaped is put into the nip of the second folding roller pair 13, and is folded in the second direction.
When a roll of paper is used as the sheet 14, the sheet is inevitably curled. Accordingly, a posture of the leading edge of the rolled or curled sheet when it is forcibly put into the nip of the paired rollers (this posture will be referred to as a "paper thrust posture") is different from that of the flat sheet of paper, such as a cut sheet or a continuous sheet of paper. Further, the paper thrust posture for the first folding roller pair 12 is often different from that for the second folding roller pair 13.
The paper thrust postures of a flat sheet of paper and upward and downward curled sheets of paper are illustrated in FIGS. 4(A) to 4(C). In FIG. 4(A), a flat sheet of paper 14 slides on the guide plate 16 and its leading edge 14a is inserted between the rollers (into a nip point) of the folding roller pair uniformly over its entire width. Accordingly, the sheet 14 can be stably folded by the folding roller pair.
In the case of FIG. 4(B) or 4(C), however, the sheet of paper 14 is put into the nip of the folding roller pair in a state that the leading edge 14a is curled upwardly or downwardly. In this case, the leading edge 14a of the sheet hits the pinch roller 12b or the drive roller 12a before it reaches the nip. If such a state occurs even at a part of the leading edge width of the sheet, a twisting force acts on the sheet 14 moving toward the nip to crease the sheet when it is folded.
Further, in the case of the large size sheet of paper, the sheet 14 is folded in the first direction, and then is folded again in the second direction. When the folded sheet is put into the nip of the second folding roller pair for the second folding, the creased side 14a of the folded sheet is slackened since the creased side 14a is more impeded than the other side 14b, or the side not creased, of the folded sheet. The folded sheet is folded again in a state that the creased side is slackened. Accordingly, the resultant folded sheet tends to have wrinkle 17a in the creased side 14a.